1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and, more particularly, to cleaning semiconductor substrates.
2. Description of the Related Art
The fabrication of semiconductor devices involves numerous processing operations. These operations include, for example, impurity implants, gate oxide generation, inter-metal oxide depositions, metallization depositions, photolithography patterning, etching operations, chemical mechanical polishing (CMP), etc. As these operations generate particles and residues, wafer surfaces need to be cleaned so as to remove particulate contaminants adhered to the surfaces of the wafer.
Particulate contaminants generally consist of tiny bits of distinctly defined material having an affinity to adhere to the surface of the wafer. Examples of particulate contaminants can include organic and inorganic residues, such as silicon dust, silica, slurry residue, polymeric residue, metal flakes, atmospheric dust, plastic particles, and silicate particles, among others. Particulate contaminants should be removed from wafer surfaces as the existence of such contaminants can have a detrimental effect on the performance of the integrated circuit devices.
Cleaning the front side (i.e., the active side or top surface) of wafers has traditionally been given a higher priority in typical wafer cleaning systems and processes because particulate contaminants adhered to the active side of a wafer can cause deleterious defects in the processing of the wafer. However, as wafer sizes have increased and/or feature sizes have decreased, certain shortcomings have been associated with the failure to adequately and properly clean and process the backside (i.e., non-active side) of wafers.
One exemplary shortcoming is the focal plane deviation phenomenon associated with reduction of depth of fields for photolithography equipments. Namely, particulate contaminants having a size ranging between about 0.5 microns and about 10 microns adhered to the wafer backside can form high and low points throughout the wafer surface. As a result, the wafer surface is (locally) tilted or distorted, thus creating a focal plane deviation. Although such deviation can be very slight, nonetheless, a number of challenges surface during photolithographic processes designed to form very small features.
In addition to creating focal plane deviation, contaminant particulates having a size ranging between about 90 nm to 0.5 micron have proven to migrate from a wafer backside to the wafer front side. For example, the migration may occur during a wet processing step and/or as the wafer is moved or otherwise handled between the processing or metrology tools. Furthermore, the backside contaminants can undesirably migrate from one process tool/step thus contaminating subsequent processes.
To eliminate such drawbacks, backside of wafers can be cleaned using a rotary type cleaning assembly or a roller type cleaning assembly. In a rotary type cleaning assembly, a wafer backside is cleaned as a brush is applied onto the wafer backside is being rotated in a specific direction. In a roller type cleaning assembly, a top brush and a bottom brush are brought into contract with the wafer front side and backside as the top brush and the bottom brush are being rotated in respective rotation directions. In either scenario, fluid is supplied onto the cleaning interface either externally or through the rotary or roller type brushes.
Unfortunately, processing the front and backside of the wafer using the roller type and rotary type brushes can have certain disadvantages. For instance, as a rotary brush rotates so as to clean and process the wafer surface, the rotary movement of the brush causes the fluid introduced into the cleaning interface to be expelled (e.g., flinging droplets) onto the portions of the wafer surface not covered by the rotary brush as well as around the inner wall of the cleaning module chamber.
With respect to roller type brushes, fluid defined at the edge of the wafer can migrate to the opposite surface of the wafer as the rollers rotate, contaminating the otherwise cleaned surface. The fluid migration phenomenon particularly hinders processing of a wafer wherein divergent process chemistries should be implemented. By way of example, as semiconductor devices only reside on the front side of the wafer, cleaning the wafer backside can be enhanced by using a more aggressive chemistry. However, due to fluid migration phenomenon, cleaning wafer surface by using truly divergent chemistries may not be achieved.
Irrespective of the brush type, expelled or migrated fluid droplets can contaminate the otherwise clean wafer surfaces, thus decreasing the quality of the cleaning operation. Furthermore, fluid droplets can be expelled onto and around the cleaning chamber, reducing the reliability of the cleaning environment. Still further, fluid migration can damage semiconductor devices on the wafer front side generating faulty wafers, thus reducing throughput.
In view of the foregoing, there is a need for a system and method for cleaning a substrate surface capable of improving the reliability and efficiency of the cleaning operations.